Don't prefetch unreachable packages

When batch prefetching we can fetch versions we know that are incompatible. In the following example, we were prefetching sentry-kafka-schemas below version 1.50.0.

```
python-rapidjson<=1.20,>=1.4
sentry-kafka-schemas<=0.1.113,>=0.1.50
```

Using a new pubgrub interface from astral-sh/pubgrub#32, we can avoid those prefetches by asking for incompatibilities that won't change anymore (those with root).

crates/uv-resolver/src/resolver/batch_prefetch.rs

@@ -1,7 +1,7 @@
 use std::cmp::min;
 
 use itertools::Itertools;
-use pubgrub::Range;
+use pubgrub::{Range, Term};
 use rustc_hash::FxHashMap;
 use tokio::sync::mpsc::Sender;
 use tracing::{debug, trace};
@@ -49,6 +49,7 @@ impl BatchPrefetcher {
         index: Option<&IndexUrl>,
         version: &Version,
         current_range: &Range<Version>,
+        unchangeable_constraints: Option<&Term<Range<Version>>>,
         python_requirement: &PythonRequirement,
         request_sink: &Sender<Request>,
         in_memory: &InMemoryIndex,
@@ -119,11 +120,22 @@ impl BatchPrefetcher {
                     }
                 }
                 BatchPrefetchStrategy::InOrder { previous } => {
-                    let range = if selector.use_highest_version(name) {
+                    let mut range = if selector.use_highest_version(name) {
                         Range::strictly_lower_than(previous)
                     } else {
                         Range::strictly_higher_than(previous)
                     };
+                    // If we have constraints from root, don't go beyond those. Example: We are
+                    // prefetching for foo 1.60 and have a dependency for `foo>=1.50`, so we should
+                    // only prefetch 1.60 to 1.50, knowing 1.49 will always be rejected.
+                    if let Some(unchangeable_constraints) = unchangeable_constraints {
+                        range = match unchangeable_constraints {
+                            Term::Positive(constraints) => range.intersection(constraints),
+                            Term::Negative(negative_constraints) => {
+                                range.intersection(&negative_constraints.complement())
+                            }
+                        };
+                    }
                     if let Some(candidate) =
                         selector.select_no_preference(name, &range, version_map, markers)
                     {

crates/uv-resolver/src/resolver/mod.rs

@@ -475,6 +475,10 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                         index,
                         &version,
                         term_intersection.unwrap_positive(),
+                        state
+                            .pubgrub
+                            .partial_solution
+                            .unchanging_term_for_package(&state.next),
                         &state.python_requirement,
                         &request_sink,
                         &self.index,